Exposure for ionizing radiation is used in many technical fields, both therapeutical and non-therapeutical, e.g. for sterilizing purposes, radiotherapy, material modification etc. Common for most applications is that there is a need for dosimetry. Dosimetry deals with the process of determining the absorbed dose to an irradiated medium. Such a determining is generally applicable for all kinds of applications and therefore not a part of any treatment or diagnosing of a human being.
In radiotherapy, patients are treated for cancer with beams of ionizing radiation. The aim is to sterilize the tumor cells while spearing the healthy tissue surrounding them as far as possible. It is imperative that the planning, delivery and quality assurance through measurements of the absorbed dose, is done with high accuracy as to ensure precisely tailored treatments. There are different modalities of radiations used, such as conventional photon beams, intensity modulated photons (IMRT), electrons, protons or light ions.
In the document “Refinements of the finite-size pencil beam model of three-dimensional photon dose calculation” by O. Z. Ostapiak et al in Med. Phys. 24(5), May 1997, pp. 743-750, photon dose calculation algorithms are presented. The dose calculation is based on a convolution of the photon fluence and a radiation dose kernel (“dose spread kernel”). Such dose calculations are frequently used for clinical dose planning, but are not possible to utilize in connection with quality assurance problems. The present invention instead relates solely to the quality assurance problem, and does not concern any treatment planning of radiation.
A variety of detectors can be used, e.g. ionization chambers, calorimeters, solid state detectors, diodes, film, or thermoluminescence detectors (TLD), for different applications such as in vitro dosimetry or in vivo dosimetry. In vitro dosimetry relates to quality assurance of the irradiation using phantoms of water or tissue equivalent plastics. In in vitro dosimetry, detectors are placed in contact with a patient.
As mentioned above, dosimetry deals with the process of determining the absorbed dose to an irradiated medium. However, the introduction of a measurement device itself, i.e. a dosimeter, will introduce changes in the radiation conditions. It is not trivial to obtain the actual absorbed dose in an irradiated object from the reading from a measurement device. Placing a detector in, or on the surface of, an object to be irradiated the reading will give a different value than what is the actual dose deposition in equivalent volumes in the irradiated object. Knowing the proportion between deposited dose in different materials at different positions enables the determination of the dose to the object. In dosimetry, correction factors are used to express such relations.
In prior art radiation devices, correction factors are typically determined experimentally in water phantoms along the central axis of the radiation field, and tabulated as correction factors for different depths for standard field sizes. An alternative approach is to calculate correction factors, e.g. through extensive and lengthy Monte Carlo simulations of the radiation transport and interactions as to determine the full spectral distribution of particles incident on the detector.
In modern radiotherapy, one modulates and shape the beam as to adjust the delivered dose to the tumor in order to spare the healthy tissue surrounding it to the outermost extent. Similar needs for customizing the irradiation field are present also in other applications. Modulating the irradiation field is done with different techniques, e.g. by dividing the treatment irradiation into partitions delivered with different amounts and field shapes, from different incident angles. Shaping the field, and their partitions, can be done either with adjustable collimator leaves, specific molded collimators or by scanning a narrow beam.